The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to multi-channel or multi-port parallel RF transmit (Tx)/receive (Rx). The present invention further relates to a calibration method and a multi-channel parallel RF transmit/receive coil assembly that improve the performance of MR imaging.
When radio-frequency signal gets transmitted/detected during MR imaging, it is the B1+/B1− fields in the object, not the driving/induced currents in the RF coil structure, that interact directly with the spin system (nuclei) of an imaged object. The concomitant E field meanwhile gives rise to RF loss in the object and dictates RF power deposition/noise. Yet the coil currents play a central role—they act as the link between Tx/Rx system electronics and the RF electromagnetic fields, and serve to implement the control of the RF electromagnetic fields for MR imaging.
One suitable perspective for analyzing Tx/Rx in a multi-port setting is as follows: (a) During Tx, predetermined RF pulses, through an array of RF power amplifier, drive currents in the RF coil structure. The currents in turn drive the B1 field, whose B1+ component excites the spins in the object out of equilibrium and causes them to precess. The RF loss associated with the concomitant E field causes RF energy deposition in the object (RF power dissipation through Joule heating and polarization damping forces=∫(σ+ωε″)|E|2dv). (b) During Rx, the rotating magnetization due to the precessing spins generates an electromotive force in the RF coil structure, which induces currents in accordance with the B1− component of the B1 field. The currents are detected by an array of RF receivers, giving rise to time signals that are used to reconstruct MR images. The RF loss associated with the concomitant E field in this case contributes to the noise in the received time signal data. Note that with both Tx and Rx, the EM field distribution and RF loss are object-dependent. Also note that the B1-spin interaction is commonly modulated in space by the gradient pulses that are played out during Tx/Rx. This spatial modulation is an integral part of the excitation/detection scheme that control the profile of the spin excitation/the mapping of the MR signal.
Optimizing the coil currents' magnitude/phase relationship, temporal modulation and spatial distribution is crucial to MR imaging performance. One key aspect for the optimization is the knowledge of B1 spatial distribution and RF power deposition associated with a coil current pattern or a source configuration, and the use of the knowledge in the optimization. Another key aspect for the optimization is a hardware infrastructure that facilitates the optimization. In addition to independent exciters and receivers, having a coil structure that supports flexible current path control is crucial. This application describes new systems and methods that address these aspects and improve MR imaging performance.
Parallel RF transmit offers unmatched capacity in containing specific absorption rate (SAR) in the mean time of realizing high quality RF excitation. However, there has not been a viable method that, under in vivo conditions, effectively taps into this capacity and realizes imaging performance enhancement with SAR reduction. In fact the substantially increased degrees of freedom inherent of parallel transmit, which are the very foundation of this capacity, tend to spur a concern that improper RF pulse or shimming calculations, or multi-channel hardware failure, may inadvertently exacerbate SAR, as opposed to reducing it. One can address this concern to some extent by monitoring RF power at the ports, where power meters are implemented to measure individual port forward and reflected power, calculate in real-time net forward power into the subject and stop scan when the overall SAR reaches a threshold. To adequately addresses safety/performance issues however one must complement existing real-time monitoring with a proactive scheme that has the ability to: 1) predict SAR consequence of an imaging sequence on the subject before the MR scan, and 2) further optimize the RF pulses to minimize SAR while still achieving the target excitation profile.
What is critically needed in general are practicable quantitative SAR prediction models that, given any set of B1 shimming coefficients or RF excitation pulses, predict the SAR consequences globally, with a volumetric average, and/or locally at locations of interest, with regional averages.
In principle, for multi-port RF pulse calculation (including B1 shimming and full fledged parallel transmit) one can explicitly minimize SAR by guiding the design with predictive models that track SAR, and in the case of hardware failure management, avert hazardous outcome by presetting a safety margin based on the same models. During an MR scan, these models can act as “software” SAR monitors, which extend the capability of the power meter reading-based global SAR monitoring scheme with the ability to predict and map SAR.
There are evidences showing that, in addition to pulse calculation, RF coil (antenna) geometry substantially impacts the performance of multi-channel or multi-port Tx and Rx. Fundamentally, a coil structure that supports flexible current path control is essential for pushing parallel Tx and Rx performances to their limits. This is consistent with the view that RF pulses modulate coil currents, which in turn drive RF electromagnetic fields, which ultimately dictating Tx/Rx performance. What is needed is an RF coil structure that forms a multi-port and substantially dense network thereby capable of accommodating and effecting sophisticated RF current distribution patterns.